The following background information is provided to assist the reader to understand the invention described and claimed below. Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
It is well known in th e air drying art that the amount of moisture suspended in any given volume of air is dependent on both the pressure and temperature of the air contained in that volume. This relationship between pressure, volume and temperature is defined by the various ideal gas laws of thermodynamics. When a quantity of air is compressed by reducing the volume it occupies, the amount of moisture that the compressed air can hold is reduced accordingly, assuming its temperature is held constant. The temperature of air, however, increases as the air is compressed, and this temperature increase enables the air to hold its moisture.
In most compressed air systems, such as those used in the railroad industry, temperature increases are undesirable. This is because the compressed air system has components downstream whose temperatures may be lower than that of the incoming moisture-laden compressed air. The moisture in such moisture-laden air tends to condense on the surfaces of these downstream components and contaminates the compressed air system and the pneumatic components that it supplies. Consequently, an aftercooler is typically inserted between the output of the compressor and the intake port of the air drying system to lower the temperature of the incoming compressed air. By lowering the temperature, the aftercooler causes some of the water vapor suspended in the air to precipitate out of the air in the form of liquid condensate. This liquid condensate is usually removed from the air drying system via well known devices such as separating chambers and coalescing elements. Despite the use of aftercoolers, separating chambers and coalescing elements, the compressed air will still hold some water vapor as it is difficult to remove this remaining moisture solely by mechanical means. Such mechanical means thus are often used in conjunction with one of the known air drying methods to remove this remaining water vapor. Depending on the specific application and environment in which it is used, an air drying method may be used alone, without the aforementioned mechanical means.
There are at least three prior art methods of drying air that are commonly used to remove water vapor. (1) Absorbent type air dryers use deliquescent desiccant that becomes liquid by absorbing moisture suspended in the air. Deliquescent air dryers typically have no moving parts and their costs are initially low. These dryers, however, exhibit limited dew point suppression--20.degree. to 30.degree. F. is common. They also require considerable maintenance, e.g., the desiccant must be periodically replaced and the system manually drained on a regular basis. (2) Absorbent type air dryers use regenerative desiccant that temporarily absorbs moisture on the surface of its molecules. The moisture temporarily accumulated by the desiccant is later removed via a stream of dried air redirected through the desiccant to purge the moisture to atmosphere. Regenerative dryers are able to achieve low dew points, but impose high costs initially and high operating costs thereafter. Their desiccant towers, in which the desiccant is housed, also must be serviced periodically. (3) Refrigeration type air dryers typically require low maintenance and impose low operating costs, but are not able to achieve low dew points. Dew points are typically limited to approximately 38.degree. F. as a minimum to prevent freeze ups. Refrigeration type dryers are used in many industries as a first step in a multi-step drying system, e.g., before drying the air in desiccant type air dryer.
Another method of drying air employs the use of semipermeable membranes to remove moisture from the air in which the moisture is suspended. These membrane type air dryers have long been used in various industries. Such membrane type air dryers typically feature a membrane fiber bundle and a containment vessel or shell in which the bundle is encased. The membrane fiber bundle is of a type that is commercially available from Bend Research, Inc. of Bend, Oreg., U.S.A.
Regarding basic operation of a membrane type air dryer, air passes through each membrane in the bundle by a combination of (i) diffusion through the pores linking the respective surfaces of a membrane and (ii) permeation through the material of the membrane. The force that drives the separation of water vapor from air is the difference between the pressure of the air on one side of a membrane and the pressure of air on the other side of the membrane. When air is compressed, the partial pressures of the various constituents in the air each increase. Water vapor, of course, is present in the stream of compressed air that flows into the inlet of the membrane housing from the source of compressed air. The partial pressure of the water vapor in the air stream flowing in the bundle will be greater than that of the atmospheric air by a factor dependent upon the compression ratio of the compressor. This difference in the partial pressure of water vapor on the inside (higher) versus that on the outside (lower) of the membranes drives the water vapor through the membranes into the sweep air space defined between the outside of the bundle and the inner wall of the containment vessel.
The vessel in which the membrane fiber bundle is encased also features a drain hole that communicates with the sweep air space. The sweep air space serves as the conduit to transport the water vapor that has permeated through the membranes to the drain hole. It is through this drain hole that the permeated water vapor is forcibly purged from the sweep air space by "sweep air". The air stream flowing through the fiber bundle causes pressure to build within its membranes. The "sweep air" that is used to purge the permeated water vapor from the vessel originates within these pressurized membranes. Composed of light gases including even hydrogen and helium that are capable of penetrating the membranes, the sweep air leaks out of the membranes and forcibly carries with it the permeated water vapor out the drain hole in the bottom of the vessel. It is for this reason that the vessel is often referred to as the sweep air containment vessel. The dried non-permeate air that emerges from the outlet of the membrane housing, of course, flows into whatever pneumatic component(s) that the membrane air dryer is intended to supply.
The membrane type air dryer is typically incorporated within a compressed air system between the source of compressed air and a reservoir or other pneumatic component to which it supplies the compressed dried air. As shown in FIG. 1, a check valve is commonly installed between the outlet of the dryer and the inlet of the reservoir to prevent air from flowing back into the dryer when the source of compressed air is unloaded (i.e., turned off). When the source of compressed air is loaded, the compressed air that flows through the fiber bundle will cause pressure to build within the membranes as described previously. It is this pressure that is the source of the sweep air. When the source of compressed air is unloaded, however, the pressure that has accumulated within the membranes is largely lost as sweep air as it is continuously vented from the drain hole of the vessel.
It is well known that on a train locomotive the compressor is controlled so that compressed air is supplied to the air system on a periodic basis. Typically, the locomotive compressor will supply compressed air once every five minutes for a time of approximately thirty seconds. A membrane type air dryer if incorporated into the air system of a locomotive would therefore be required to operate according to this duty cycle. During the thirty second period when the compressor is loaded (i.e., the drying phase of the duty cycle), the compressed air that would flow into the fiber bundle would cause pressure to build within the membranes as described previously. When turned off during the inactive phase of its duty cycle, the compressor would be unloaded for such a long time that much, if not most all, of the pressure built up within the membranes would be lost as sweep air. Consequently, when again turned on for the next thirty second period, the compressor would spend too much of that time span on merely re-pressurizing the membranes of the fiber bundle.
The primary disadvantage to this setup is that sweep air is continuously vented from the vessel of the membrane type air dryer. It would therefore be desirable to devise a system that prevents excessive loss of air as sweep air from the sweep air containment vessel of a membrane type air dryer.